1. Field of the Invention
The present invention relates to a BGA (Ball Grid Array) package substrate for mounting a semiconductor device such as LSI.
2. Prior Art
In recent years, BGA packages have attracted attention as surface mounting packages for mounting semiconductor devices.
FIG. 4 a schematically shows the structure of a conventional BGA package.
As shown in FIG. 4a, a BGA package 101 is designed to mount a semiconductor device 103 such as an LSI chip on one surface of a printed wiring board 102 and to form solder balls 104 as outer electrodes on the back surface.
In this BGA package 101, bonding wires 105 connected to electrode pads 103a on the semiconductor device 103 are connected to a conductive pattern 106 formed on the printed wiring board 102. This conductive pattern 106 is connected to lands 108a of a conductive pattern 108 formed on the back surface of the printed wiring board 102 through via holes 107 passing through the printed wiring board 102, whereby the electrode pads 103a of the semiconductor device 103 are connected to the solder balls 104.
The BGA package 101 with such a structure can be used as CSP (Chip Size/Scale Package) with a small packaging area to permit high-density surface mounting of various types of semiconductor devices 103.
However, this type of conventional BGA package 101 had the problem that a solder resist layer 109 made of a resin such as polyimide on the back surface of the printed wiring board 102 as shown in FIG. 4b expands or contract during temperature cycling tests, for example, to cause a stress in the solder balls 104, which invites microcracks lowering the reliability of connection to a mother board, because the solder resist layer 109 is designed to come into contact with the solder balls 104 in the conventional BGA package 101.
In order to solve this problem, a BGA package was proposed wherein the diameter of the lands 108a of the conductive pattern 108 formed on the back surface of the printed wiring board 102 is reduced and the solder resist layer 109 is kept out of contact with the lands 108a, as shown in FIG. 4c, for example.
However, the conventional example shown in FIG. 4c had the problem that an opening 110 formed in the solder resist layer 109 to receive solder balls 104 should have a diameter comparable to the diameter of the solder ball 104, which hinders fine-pitch packaging.
Another problem of this conventional example is that the printed wiring board 102 must have great rigidity and thickness to reliably retain the lands 108a because the solder resist layer 109 is not designed to overlap the periphery of each land 108a for receiving a solder ball 104, whereby it was difficult to construct with a thin substrate made of a resin such as polyimide.
If such a structure as shown in FIG. 4c is applied to a thin substrate with low rigidity made of a resin such as polyimide, the pattern of the land 108a may be peeled or a plating layer on the surface of the land 108a may be peeled. As a result, it was difficult to obtain a thin and light BGA package with such a structure.
The present invention was made to solve these problems of the prior art, particularly with the object of providing a BGA package substrate capable of forming a thin and light BGA package which causes no crack in solder balls during temperature cycling tests and which permits fine-pitch packaging.
In order to attain the above object, the invention of claim 1 provides a BGA package substrate comprising an electrode for connection to an IC and an electrode for connection to a mother board on a circuit board consisting of a conductive circuit formed on an insulating base wherein at least said electrode for connection to a mother board is in the form of a solder ball, characterized in that an opening is formed in said insulating base in such a manner that the periphery of a connecting land to be formed the solder ball-like electrode thereon of said conductive circuit may be overlapped by said insulating base and that an end of said opening in said insulating base is tapered.
According to the invention, the tapered end of the opening in the insulating base allows the area of the end of the opening to be greater than the area of the connecting land exposed at the bottom, with the result that the solder ball-like electrode can be kept out of contact with the end of the opening of the insulating base even if the opening is formed in the insulating base to leave an overlap on the periphery of the connecting land.
Accordingly, the invention prevents any microcrack due to stress during temperature cycling tests in the solder ball-like electrode.
There is no possibility that the conductive pattern on the connecting land portion might be peeled or the plating layer on the surface of said land might be peeled even if a thin resin is used as an insulating base, because the connecting land is retained by the insulating base.
In this case, the invention is effective when the angle of the taper is 70xc2x0 or less with the connecting land of the conductive circuit.
According to the invention, contacts between the end of the opening of the insulating base and the solder ball-like electrode can be more reliably prevented.